An electric power steering apparatus applying a steering assist force to a steering apparatus of a motor vehicle on the basis of a rotating force of a motor is structured such that the steering assist force is applied to a steering shaft or a rack shaft by applying a driving force of the motor by means of a transmission mechanism such as gears, a belt or the like via a speed reducer. A description will be given of a brief structure of the electric power steering apparatus mentioned above with reference to FIG. 1. A shaft 102 of a steering wheel 101 is coupled to a tie rod 106 of a steered wheel via reduction gears 103, universal joints 104a and 104b and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 107 detecting a steering torque of the steering wheel 101, and a motor 108 assisting a steering force of the steering wheel 101 is coupled to the shaft 102 via the reduction gears 103.
It is necessary to correctly execute the control in such a manner that the motor 108 of the electric power steering apparatus having the structure mentioned above outputs a desired torque corresponding to a handle operation by a driver. A vector control corresponding to one of typical control methods for controlling the motor 108 of the electric power steering apparatus is disclosed, for example, in Japanese Patent Application Laid-open No. 2001-18822A. FIG. 2 is a block diagram showing a basic structure of a control system disclosed in Japanese Unexamined Patent Publication No. 2001-18822.
Describing the control apparatus, a torque command value Tref and a rotation angle θ are inputted to a current command value calculating section 204. The torque command value Tref is calculated by a torque command value calculating section (not shown) on the basis of a steering torque Tr detected by the torque sensor 107, and the rotation angle θ corresponds to an electrical angle of the motor 108. The current command value calculating section 204 calculates a current command value Iqref of a q-axis component and a current command value Idref of a d-axis component. Generally, the current command value Iqref is in proportion to the torque command value Tref, and the current command value Idref equals to “0”. On the other hand, as an angle detecting device for detecting the rotation angle θ of the motor 108, a resolver 201 is installed. A signal output from the resolver 201 does not directly indicate the rotation angle θ, the rotation angle θ is calculated in a rotation angle detecting circuit 202 such as a resolver-digital conversion circuit or the like.
The present block structure diagram exemplifies a feedback control system, and it is necessary to execute a feedback control by detecting actual motor currents Ia, Ib and Ic of the motor 108, with respect to the current command values Iqref and Idref mentioned above. Specifically, the motor currents Ia and Ic are detected in the current detecting devices 205-1 and 205-2, and the motor current Ib is calculated as “Ib=−(Ia+Ic)”, in a subtraction section 207-3 on the basis of a relation of “Ia+Ib+Ic=0”. Next, they are converted into motor currents Iq and Id by a three-phase/two-phase conversion section 206 for a vector control. The rotation angle θ mentioned above of the motor is utilized for the conversion. The motor currents Iq and Id are respectively feedbacked to the subtraction sections 207-1 and 207-2, a deviation ΔIq between the current command value Iqref and the motor current Iq is calculated by the subtraction section 207-1, and a deviation ΔId between the current command value Idref (normally Idref=0) and the motor current Id is calculated by the subtraction section 207-2.
The deviations ΔIq and ΔId are inputted to a proportional integral (PI) control section 208, and voltage command values Vdref and Vqref are outputted from the PI control section 208. Further, since it is necessary that the actual motor 108 supplies a three-phase current, the voltage command values Vdref and Vqref are converted into three-phase voltage command values Varef, Vbref and Vcref by a two-phase/three-phase conversion section 209. A PWM control section 210 generates a PWM control signal on the basis of the voltage command values Varef, Vbref and Vcref, and an inverter circuit 211 supplies the current to the motor 108 on the basis of the PWM control signal, and supplies the motor currents Ia, Ib and Ic in such a manner that the deviations ΔIq and ΔId with respect to the current command value Iqref and Idref become 0.
A feature of the vector control mentioned above exists in a matter that a control calculation is executed by two phases converted into the d-axis and the q-axis in the middle of the vector control in spite that the motor current of the three-phase motor corresponding to the controlled subject is detected by three phases Ia, Ib and Ic, and the motor currents Ia, Ib and Ic are supplied by the inverter circuit. Converting into the d-axis and the q-axis as mentioned above is advantageous in the case of independently controlling the torque, however, the following problems exist.
For example, in the case that a resistive component of a motor winding corresponding to a motor parameter of the motor 108 is changed on the basis of a heat generation of the motor winding, if a heat radiating condition or the like is different in each of the phases, for example, if a winding resistance Ra of the a-phase motor becomes (Ra+ΔR) in comparison with the other phases, a motor current Ia is changed to (Ia+ΔI). On the contrary, if the winding resistances of the b-phase and c-phase motors are not changed, and the same currents Ib and Ic as before are applied, a conversion as described in numerical expression 1 is executed in the three-phase/two-phase conversion section 206.
                                                                        [                                                                            Id                                                                                                  Iq                                                                      ]                            =                            ⁢                              [                                                                                                    -                                                  cos                          ⁡                                                      (                            θ                            )                                                                                                                                                              -                                                  cos                          ⁡                                                      (                                                          θ                              -                                                              2                                ⁢                                                                                                                                  ⁢                                                                  π                                  /                                  3                                                                                                                      )                                                                                                                                                              -                                                  cos                          ⁡                                                      (                                                          θ                              +                                                              2                                ⁢                                                                                                                                  ⁢                                                                  π                                  /                                  3                                                                                                                      )                                                                                                                                                                                                  sin                        ⁡                                                  (                          θ                          )                                                                                                                                    sin                        ⁡                                                  (                                                      θ                            -                                                          2                              ⁢                                                                                                                          ⁢                                                              π                                /                                3                                                                                                              )                                                                                                                                    sin                        ⁡                                                  (                                                      θ                            +                                                          2                              ⁢                                                                                                                          ⁢                                                              π                                /                                3                                                                                                              )                                                                                                                    ]                                                                                                      ⁢                              [                                                                  ⁢                                                                                                    Ia                        ⁢                                                                                                  +                                                                                                  ⁢                                                  Δ                          ⁢                                                                                                          ⁢                          Ia                                                                                                                                                Ib                                                                                                  Ic                                                                      ]                                                                                        =                            ⁢                              [                                                                                                                                                          Δ                            ⁢                                                                                                                  ⁢                                                          Ia                              ·                              cos                                                        ⁢                                                          (                              θ                              )                                                                                ⁢                                                                                                          -                                                ⁢                                                                                                                                                              ⋯                                                                                                                                                                                                                                                                                            Δ                            ⁢                                                                                                                  ⁢                                                          Ia                              ·                              sin                                                        ⁢                                                          (                              θ                              )                                                                                ⁢                                                                                                          +                                                ⁢                                                                                                                                                              ⋯                                                                                                                                                                                  ]                                                                        [                  Numerical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          1                ]            
In other words, when the parameter change generated in the a-phase is diffused to elements in the d-axis and the q-phase, particularly returned to the voltage command values Varef, Vbref and Vcref by the two-phase/three-phase conversion section 209 after passing through the PI control section 208, an error compensation value by the a-phase parameter change is embedded to the b-phase and c-phase voltage command values. Since the parameter change is generated only in the a-phase originally, the error should be compensated only in the a-phase, however, the error compensation value is contained also in the b-phase and the c-phase. Further, if a rotation speed (an angular velocity ω)) of the motor is high, the error (ΔIa·cos θ, ΔIa·sin θ) is changed to a high speed in the d-axis and the q-axis, and in the case that the current control section can not follow, a torque ripple is generated.
Further, in the control of the three-phase motor as shown in FIG. 2, there is employed a control system which converts the three-phase motor currents Ia, Ib and Ic corresponding to the controlled subject into the d-axis and q-axis currents Id and Iq once in the control system so as to determine the control amount, and again returns to the three phases. Accordingly, in the case that the change generated in a partial phase in three phases, for example, the change of the resistance value due to a temperature change of the winding resistance of the motor is different in each of the phases, the change amounts in the phases are mixed at a time of being converted into the d-axis and the q-axis, in the control system mentioned above, so that it is impossible to execute the control which uniquely corresponds to each of the phases. As a result, an error of the current control is generated, and it is impossible to converge the error. Accordingly, there is generated a problem that a steady-state deviation is left.
Further, in the control of the electric power steering apparatus, there exists a control system which executes a current control individually in three phases as shown in Japanese Patent Application Laid-open No. 2000-118424 A, without adopting the system converting the three-phase motor into the d-axis and the q-axis. However, totally four conditions comprising a condition that a total of the three-phase currents is zero (Ia+Ib+Ic=0) in addition to the current control condition in each of the phases exist as a condition satisfied by the controlled subject with respect to the control amounts comprising the motor currents Ia, Ib and Ic, the error is accumulated, and is not said to be preferable as the control system.
If the three-phase motor is controlled by the vector control using the d-axis and q-axis conversions as mentioned above, it is impossible to execute the unique control in each of the phases, for example, with respect to a tolerance in each of the phases of an inductance value or a resistance value of the winding of the three-phase motor, or the dispersion in each of the phases of the change amount of the resistance value due to the temperature change, and the stationary-state deviation of the current control is left. Further, in the individual current control of three phases, since four conditions to be satisfied exist in spite of three control amounts, the error is accumulated and the control system is not said to be preferable.
Further, in the electric power steering apparatus as shown in FIG. 1, the control of the motor 108 is important, however, relations shown in the following numerical expressions 2, 3 and 4 are established, for example, in the voltage and the current of the three-phase motor.Van=Va−Vn=EMFa+La·(dIa/dt)+Ra·Ia  [Numerical Expression 2]Vbn=Vb−Vn=EMFb+Lb·(dIb/dt)+Rb·Ib  [Numerical Expression 3]Vcn=Vc−Vn=EMFc+Lc·(dIc/dt)+Rc·Ic  [Numerical Expression 4]
In this case, a description will be given of definitions relating to the voltage and the current of the motor, and the inductance value and the resistance value of the motor winding with reference to FIG. 3.
Reference symbols Van, Vbn and Vcn denote a voltage between a neutral point of the motor and each of the phases (a-phase, b-phase and c-phase). Reference symbols Va, Vb, Vc and Vn denote a voltage between a ground earth and each of the phase terminals and a voltage between the ground earth and the neutral point, in other words, a potential. Reference symbols Ia, Ib and Ic denote respective phase currents of the motor. Reference symbols La, Lb and Lc and reference symbols Ra, Rb and Rc denote an inductance value and a resistance value in the respective phases of the motor winding.
In this case, if the neutral point potential Vn is changed, the voltages Van, Vbn and Vcn in the respective phases are changed, and if the current control is not executed on the basis of the error, a torque ripple is increased.
In response to the problem mentioned above, for example, Japanese Patent Application Laid-open No. 2002-84758A discloses a control system stabilizing the neutral point, in connection with the motor control of the structure provided with a power source in the neutral point.
However, there is nothing which discloses the control system stabilizing the neutral point in the motor control of the system in which the power source is not provided in the neutral point. If the potential of the neutral point is unstable as mentioned above, the torque ripple of the motor becomes undesirably larger. Further, if a specific sensor is necessary for a control for stabilizing the potential of the neutral point, a cost is increased, and if a specific control is executed, a processing load of a CPU is increased.
The present invention is made by taking the circumstances mentioned above into consideration, and a first object of the present invention is to provide a control apparatus of an electric power steering apparatus which can execute a motor control in such a manner that a robust control is generated with respect to a parameter change in each of phases in the case that a polyphase motor is used in a motor-drive power steering apparatus, and has a reduced torque ripple. Further, a second object of the present invention is to provide a control apparatus of an electric power steering apparatus which can execute a stable motor control without leaving an error in each of the phases with respect to a dispersion of a control amount in each of the phases, in a control of a three-phase motor.
Further, a third object of the present invention is to provide a control apparatus of an electric power steering apparatus which can achieve a handle operation having a good feeling by stabilizing a neutral point of a polyphase motor in which a power source is not installed at the neutral point so as to make it possible to control the motor with a reduced torque ripple and noise, in a control system in which a control process is simple and a burden on a CPU or the like is small, without installing any specific sensor.